1. Field of the Invention
This invention relates to an image sensor for use in such devices as facsimile machines and scanners, and more particularly to an image sensor capable of accurately producing a read signal.
2. Discussion of the Related Art
Thin film transistor (TFT)-driven image sensors are known contact type image sensors. The TFT-driven image sensor projects image data of an original or the like on a one-to-one basis and converts it into an electric signal. The projected image is divided into a number of picture elements. The electric charges generated at the respective photo-receiving elements are temporarily stored by respective load capacitors for a block comprising a predetermined number of photo-receiving elements using thin film transistor switching elements. The electric charges are read as electric signals sequentially and chronologically at speeds of from several hundreds of KHz to several MHz. In the TFT-driven image sensor, the operation of the TFTs allows a single drive IC to read the image data, thus, reducing the number of drive ICs.
The TFT-driven image sensor, whose equivalent circuit diagram is shown in FIG. 6, comprises a line-like photo-receiving element array 51 whose length is substantially the same as the width of an original, an electric charge transfer unit 52 consisting of a plurality of thin film transistors TN,n, whose number corresponds to that of photo-receiving elements 51' on a one-to-one basis, and a multilayer interconnection 53.
The photo-receiving element array 51 is divided into a plurality of N blocks, each block consisting of a plurality of n photo-receiving elements 51'. The plurality of n photo-receiving elements 51' can be expressed equivalently as a photodiode PDN,n. Each photo-receiving element 51' is connected to the drain electrode of a corresponding thin film transistor TN, n. The source electrode of each thin film transistor TN,n is connected to a corresponding common signal line 54 (n lines) and a corresponding load capacitor Cn. Thus, every photo-receiving element block is connected to a load capacitor Cn and a common signal line 54 through the matrix-like connected multilayer interconnection 53.
The gate electrode of each thin film transistor TN,n is connected to a gate pulse generating circuit (not shown). A single line connects each thin film transistor in a block so that each thin film transistor in a block can be turned on simultaneously.
The photoelectric charges generated at each photo-receiving element 51' are stored by the parasitic capacitance of each photo-receiving element 51' and by the overlap capacitance between the drain and gate of the thin film transistor for a predetermined time period. The photoelectric charges are then sequentially transferred to and stored by the load capacitor Cn for every block using the thin film transistor TN,n as a transfer switch. When a gate pulse .phi.G1 from the gate pulse generating circuit is applied to turn on thin film transistors T1,1 to T1,n of the first block, photoelectric charges generated and stored by the respective photo-receiving elements 51' of the first block are transferred to and stored by the respective load capacitors Cn.
The potentials of the respective common signal lines 54 are changed by the amount of electric charges stored by the load capacitors. The changed voltage values are used to sequentially turn on analog switches SWn within the drive IC 55 to transfer the voltages to an output line 56 chronologically.
By turning on thin film transistors T2,1 to T2,n, through TN,1 to TN,n of the second to Nth blocks by gate pulses .phi.G2 to .phi.Gn, the electric charges stored by the photo-receiving elements are transferred for every block and sequentially read. This sequential operation allows image signals to be obtained for every line of the original in a main scanning direction, and the image signals covering the entire original can be obtained by repeating this operation, while advancing the original documents by an original document conveying means (not shown) such as a roller described in Japanese Patent Unexamined Publication No. Sho. 63-9358.
There is a drawback in the image sensor thus constructed. If the load capacitors C1 to Cn are located intensively at one place, as shown by the equivalent circuit diagram in FIG. 6, the length of a line connecting each photo-receiving element and each load capacitor Cn varies with the photo-receiving element block, and this causes variations in resistance, inductance component, and the like, thereby affecting the accuracy with which the electric charges are read. That is, the longer the lines from the photo-receiving elements are, the larger the resistance and the like becomes, thus conversely reducing the electric charges stored by each load capacitor Cn. As a result, even with the same amount of light received, their outputs become erratic, thereby presenting the problem of impairing the output accuracy of the image sensor.